The invention relates generally to marine seismic prospecting and, more specifically, to devices and methods for controlling the lateral position of streamer cables towed underwater behind survey vessels.
In marine seismic exploration, instrumented cables, known as streamers, are towed underwater by a survey vessel. The streamers are outfitted with a variety of electronic devices, including hydrophones that detect seismic signals transmitted into the water and reflected off geologic structures beneath the sea floor.
Devices known as cable-leveling birds are attached to a streamer at intervals along its length to control the streamer's depth. The birds are equipped with adjustable diving planes, generally referred to as wings, each having a pitch axis about which the wings can be pivoted by a motor to generate the lift needed to maintain the cable at a desired depth. Most commonly, the birds are rotatably attached to the streamer and weighted to hang pendulously from the cable with the pitch axis of the wings below the cable. These birds are effective depth-control devices.
It is not uncommon for a survey vessel to tow six or eight or more streamers of lengths up to 12 km. Because the costs of lost survey time and of replacing a damaged or lost streamer are so high, it is important that the streamers not become entangled during their deployment. Entanglement is more likely to occur in the presence of strong cross currents or while the survey vessel is turning to make another pass across the survey zone. To help avoid entanglement in turns, for example, each streamer is often operated at a different depth. While this technique provides some measure of entanglement control, it also subjects the cables to potentially strong shear layers of current that vary considerably with depth, possibly increasing the risk of entanglement. Generally the most satisfactory way to avoid entanglement with conventional systems is to steer the vessel through wide turns and to overspace the streamers from each other. But these techniques increase cost and reduce the precision of the seismic image.
Paravanes and other devices are used to separate the streamers at their head ends close to the survey vessel. But lateral streamer control and streamer position predictability diminish as cable tension lessens down the lengths of streamers. The wake created by the seismic vessel creates a phenomenon known as “trouser” effect on the array. The streamers fan out port and starboard, creating a large void in the seismic coverage directly aft of the vessel. The streamers assume the shape of trousers. These voids must be resurveyed on subsequent passes known as “in-fill.” In-fill can increase the cost of seismic surveying by up to 20%. Lack of repeatability in processes and positional inaccuracies can reduce the quality of the seismic data and increase the cost by necessitating in-fill. Thus, there is a need to provide a technique for lateral streamer positioning to reduce the cost of operation and to improve the quality of the resultant seismic image.
Today's state-of-the-art seismic vessels have the capacity to deploy, tow, and recover up to 18 streamers. Existing deployment schemes limit the degree to which streamers can be simultaneously deployed, which greatly increases the cost of operation. There is a need to provide for lateral streamer control during the deployment and recovery phases to support simultaneous streamer operation without entanglement.